Wing-Kit Choi

Tunable-Focus Cylindrical Liquid Crystal Lenses

A cylindrical lens is demonstrated using two planar substrates and a homogeneous liquid crystal (LC) layer. The top substrate has slit electrodes on its outer surface and the bottom has a transparent electrode on its inner surface. An applied voltage across the cell induces a cylindrical-like gradient refractive index within the LC layer which causes light to focus. The focal length is tunable by the applied voltage. Such a cylindrical lens can be easily fabricated with wide aperture size.

Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell

A fast response nematic liquid crystal (LC) modulator using dual fringe field switching (DFFS) mode is demonstrated. Both top and bottom substrates have pixel and common electrodes to generate complementary fringing fields. The cell consists of a vertically aligned LC layer whose dielectric anisotropy is positive. In a voltage-on state, self-imposed thin LC walls near the center of the slits and electrodes are formed and the surrounding LC molecules exert a strong restoring force. As a result, submillisecond gray-to-gray response time is achieved. Two DFFS cells are used as examples to illustrate the design principles for display and photonic applications.

Design and fabrication of birefringent nano-grating structure for circularly polarized light emission

Three different nano-grating structures are designed as phase retarders that can transform linearly polarized light to circularly polarized emission for the wavelengths of 488 nm, 532 nm and 632.8 nm, respectively. Gold based nano-grating structures with various periods are fabricated by utilizing laser interference lithography. The ellipticity of all circularly polarized emission can reach around 90% such that the structure has great potential in the applications of three-dimensional (3D) display. The effects of the slit width and metal thickness modulations are simulated by rigorous coupled wave analysis (RCWA) method. Besides, the field intensity and phase of the transmitted TM and TE waves are also simulated to understand their polarization characteristics.

Plasmon-Enhanced Photoluminescence of an Amorphous Silicon Quantum Dot Light-Emitting Device by Localized Surface Plasmon Polaritons in Ag/SiOx:a-Si QDs/Ag Sandwich Nanostructures

We investigated experimentally the plasmon-enhanced photoluminescence of the amorphous silicon quantum dots (a-Si QDs) light-emitting devices (LEDs) with the :a-Si QDs/Ag sandwich nanostructures, through the coupling between the a-Si QDs and localized surface plasmons polaritons (LSPPs) mode, by tuning a one-dimensional (1D) Ag grating on the top. The coupling of surface plasmons at the top and bottom :a-Si QDs interfaces resulted in the localized surface plasmon polaritons (LSPPs) confined underneath the Ag lines, which exhibit the Fabry-Perot resonance. From the Raman spectrum, it proves the existence of a-Si QDs embedded in Si-rich film ( :a-Si QDs) at a low annealing temperature (300°C) to prevent the possible diffusion of Ag atoms from Ag film. The photoluminescence (PL) spectra of a-Si QDs can be precisely tuned by a 1D Ag grating with different pitches and Ag line widths were investigated. An optimized Ag grating structure, with 500 nm pitch and 125 nm Ag line width, was found to achieve up to 4.8-fold PL enhancement at 526 nm and 2.46-fold PL integrated intensity compared to the a-Si QDs LEDs without Ag grating structure, due to the strong a-Si QDs-LSPPs coupling.

Fast-Response Single Cell Gap Transflective Liquid Crystal Displays

A single cell gap transflective liquid crystal display (TR-LCD) using dual fringing field switching mode is proposed, in which a positive dielectric anisotropy liquid crystal is vertically aligned and driven by fringing fields from both substrates. By optimizing the electrode width and gap of the transmissive and reflective regions, this TR-LCD exhibits a fast response time, high optical efficiency, single gamma curve, and wide viewing angle. Fast response time enables color sequential operation using red, green, and blue light-emitting diodes without noticeable color breakup. Potential application of this TR-LCD for sunlight readable mobile displays is emphasized.

Ultra-Bright Heads-Up Displays Using a Method of Projected Color Images by Combination of LEDs and Polymer-Dispersed Liquid Crystals

A heads-up display (HUD) commonly uses liquid crystal technology to generate images. However, the intensity of the light decreases significantly due to passing through polarizers. Therefore, a polarizer-free display is a superior approach to utilize light more efficiently. We demonstrate the feasibility of using polymer dispersed liquid crystal (PDLC), which consists of polymer and liquid crystal, as an optical shutter to fabricate a simple see-through projected display device. The unique electro-optical characteristics of PDLC make it suitable to define the projected image shape. In our device, we used a 36 watts collimated light emitting diode as a backlight source so that the projected image can also be seen clearly under daylight. Since PDLC does not require/utilize polarizers, it achieved high light efficiency ( ~ 70%). In this paper, we showed promising results of projected images with various colors (red, green, and blue) that can be applied to see-through projected displays. From our experimental results, the see-through projected display device by PDLC can achieve a contrast ratio of 1000:1 and the response time is approximately 15 ms. The driving voltage is less than 25 V and brightness is over 3000 nit.

Narrow-band amplified photoluminescence of amorphous silicon quantum dots via the coupling between localized surface plasmon and Fabry-Pérot cavity modes

Abstract. We experimentally investigate the multifold intensity enhancement and spectral narrowing of photoluminescence (PL) from amorphous silicon quantum dots (a-Si QDs) embedded in a silicon-rich SiOx film of the Ag/SiOx:a-Si QDs/Au plasmonic nanocavity, through the resonance coupling between the localized surface plasmon (LSP) mode and the Fabry-Pérot (FP) cavity mode, by tuning a one-dimensional (1-D) Ag grating on the top. The LSP resonance can be precisely tuned by adjusting the Ag line widths of the 1-D Ag grating. It is found that the LSP mode strongly couples with the FP cavity mode, resulting in a narrower emission line width and a larger PL enhancement. An optimized Ag grating structure is found to exhibit a narrow emission line width of 15 nm and 2.77-fold enhancement in the PL peak intensity, as compared to an SiOx:a-Si QDs/Au structure without 1-D Ag grating, due to the strong resonance coupling between the two modes.

A novel fabrication of fiber Bragg grating in hollow-core fiber with HPDLCs

An electrically controllable fiber Bragg grating (FBG) for working in the communication band is demonstrated by utilizing holographic polymer-dispersed liquid crystal (HPDLCs). PDLC is infiltrated in hollow-core fibers which are about 2μm and 5μm by means of capillarity. For the purpose of periodically separating polymer and liquid crystal to form an FBG, a two-beam interference system based on an argon laser (wavelength: 364nm) is used. To reduce coupling loss, we directly connect single-mode fibers (SMFs) to input and output ports. A maximum transmission loss dip of approximately 5-dB band rejection is obtained. After the cladding diameter is etched by buffered oxide etchant (BOE) solution to 12μm and a 150V external voltage is applied, a dip shift by ~ 15nm is measured

Vertically-Aligned Polymer Stabilized Liquid Crystals (VA-PSLC) with a Curing Voltage for Fast-Response Wavelength-Tuning Applications

We present experimental results obtained for a Vertically-Aligned Polymer Stabilized Liquid Crystal (VA-PSLC) with a curing voltage. The curing voltage was found to help reduce the otherwise strong scattering effect of the VA-PSLC. This liquid crystal mode was placed inside a Fabry-Perot cavity to achieve a wavelength tunable filter. Wavelength tuning range was found to decrease as curing voltage increased, which is consistent with what we expected since molecules had a larger pretilt angle when the curing voltage was higher. Shortening of response time (>10X) was found as polymer concentration increased since the polymer effect helped improve the response speed. These filters can have potential applications in wavelength tuning applications (e.g. WDM) in telecommunication systems where high speed is desirable. This new liquid crystal mode of VA-PSLC with a curing voltage can also find applications where both fast response time and pure phase modulation are desirable.

Fast-response & Polarization-independent Optical Shutter Using Nano-PDLC Inside a Fabry-Perot Cavity

In this paper, we present experimental results obtained for a device incorporating a nano-PDLC inside a Fabry-Perot cavity. Our aim is to produce a fast-response and polarization-independent light shutter with potential applications in e.g. telecommunication systems since nano-PDLC is known for its fast response time. In our chosen example of nano-PDLC with 70 wt% polymer and 30 wt% liquid crystal (E7), we obtained a light shutter with contrast ratio = ∼7.5 under an applied voltage of ∼50 V. Response time is ∼2 to 3ms and transmission is ∼20%. All these device performances, such as operation voltage, transmission and response time can be improved in future by improving the cavity performance and using a faster liquid crystal material. Moreover, comparison in performance is also made for devices with different concentrations of polymer and liquid crystals. Discussion on the residual scattering effect of the nano-PDLC inside the Fabry-Perot cavity is also included. It is interesting to observe that the undesirable effect of the residual scattering of nano-PDLC at low voltages may actually help improve the contrast ratio and/or lower the operation voltage of the device.